In recent years, demands for a solar cell such as a monocrystalline silicon solar cell, a polycrystalline silicon solar cell and an amorphous silicon solar cell are more and more increasing. These solar cells are utilized mainly for home electrical power generation, commercial electrical power generation and the like. As other solar cells, a CIS solar cell, a CdTe solar cell, a dye-sensitized solar cell, an organic thin film solar cell and the like have been developed, and these are also being put into practical use.
In a solar cell, a glass substrate with a conductive film is used as the electrode substrate. Here, a soda lime glass is generally used as the glass substrate, because of its advantage in view of production cost and versatility. Also, as the conductive film, a transparent conductive film such as fluorine-doped tin oxide (FTO) and tin-doped indium oxide (ITO) is used. Above all, FTO is chemically and thermally stable, though inferior to ITO in the resistivity, and is expected to produce an effect of, for example, confining light by the uneven profile of the film surface or enhancing the electrical conductivity by the increase in the surface area, and therefore, FTO is widely used as an electrode substrate for a dye-sensitized solar cell or amorphous silicon solar cell (see, for example, Patent Document 1 and Non-Patent Document 1).
In general, for the production of an FTO film, a thermal chemical vapor deposition (thermal CVD) method is used in view of good film formability and low cost. Specifically, the film is formed by causing a mixed gas of compounds containing tin and fluorine to undertake a thermal decomposition reaction on a glass substrate heated at about 400° C. or more. In this connection, the thermal CVD method includes an on-line CVD method of forming a film in a sheet glass production line by utilizing the heat thereof, and an off-line CVD method of cutting once-cooled glass into a predetermined dimension and re-heating it to form a film.
Meanwhile, with recent popularization of mobile electronic appliances, a solar cell is also coming into use as a power source thereof, in addition to the conventional battery. In use for a mobile electronic device, a solar cell is required to be more reduced in the thickness and weight than the conventional solar cell installed outdoors and used for home or commercial power generation. In particular, a dye-sensitized solar cell exhibits high energy conversion efficiency for indoor light compared with a crystalline silicon solar cell, and the demand for this solar cell is increasing.
A dye-sensitized solar cell consists of, for example, a glass substrate with a conductive film, a porous oxide semiconductor electrode composed of a porous oxide semiconductor layer (mainly titanium oxide layer) formed on the glass substrate with a conductive film, a dye such as Ru dye adsorbed to the porous oxide semiconductor electrode, an iodine electrolytic solution containing iodine, and a counter electrode substrate having formed thereon a catalyst film and a conductive film. Here, in order to prevent leakage of the iodine electrolytic solution filled between the glass substrate with a conductive film and the counter electrode substrate, the outer peripheral edges of the glass substrate with a conductive film and the counter electrode substrate are sealed with a sealant such as resin and low melting-point glass, e.g., lead glass, bismuth borate glass.